Organic semiconductors, conjugating polymers in particular, have electronic properties of inorganic semiconductors and engineering properties of plastic. They have been employed in many electro-opto applications, including light emitting diodes, solar cells, sensors, transistors and devices for data storage [J. M. Shaw, P. F. Seidler, IBM Journal of Research & Development, 45(1), 3(2001)].
A simple organic semiconductor device may consist of one layer of electro-opto active organic material sandwiched between two electrodes. However, the inherent charge mobilities of organic semiconductors are generally much lower than that of inorganic semiconductors, which ultimately results in difficulties in configuring a PN junction device with organic semiconductors. For example, in configuring a photovoltaic device, an inorganic semiconductor based solar cell can yield an efficiency of 15% or higher with a simple PN junction structure (as shown in FIG. 1). But, for a similarly structured device made of organic semiconductors, the efficiency is only about 1% [C. T. Tang, Applied Physics Letter, 48(86), 183 (1986)].
To facilitate charge separation and transport in organic semiconductor based devices, various heterojunction systems have been proposed and experimented. These systems include a physical blend of a p-type polymer with a n-type polymer [G. Yu and A. J. Heeger, Journal of Applied Physics, 78(7), 4510(1995)], a blend of fullerene or its derivatives into conducting polymers [N. S. Sariciftci, L. Smilowitz, A. J. Heeger, and F. Wudl, Science 258, 1474 (1992).], and a more recent hybrid system of inorganic nanoparticles or nano-rod into conducting polymers [W. H. Huynh, J. J. Dittmer, and A. P. Alivisatos, Science, 295: 2425(2002)]. These hetrojunction systems have demonstrated some improvements comparing to a simple bi-layer configuration. However, the efficiency and operation stability of these hetrojunction systems are still far below what have been achieved with inorganic semiconductors based devices, not to mention the fabrication complexity of fullerence derivatives or nano-rod of inorganic semiconductors. This is mainly because of the unsolved fundamental problems with organic semiconductor materials in these systems. The systems are still in disorder in the nano-scale or the molecular level due to phase separation and discontinuous charge transport media. In either the fullerence derivative-doped polymer system or the inorganic semiconductor nanoparticle (or nano-rod) polymer hybrid system, in order to be colleted at the two electrodes, charges still have to hop from one molecule (or one particle) to another repeatedly. In this mode of transport, two molecules have to be really close and well oriented in order to allow for efficient charge hopping. Moreover, in these blended systems, which behave like “meat-balls in spaghetti”, phase separation may occur in fabrication stage and in operation stage. The phase separation may cause dimensional instability and performance degradation. Therefore, how to maximize charge transport and enhance dimensional stability is a critical issue for improve the performance of an organic semiconducting material based device, solar cell in particularly.